Pure gasoline used as automobile-engine fuel includes light gasoline including a hydrocarbon, as a main component, such as heptane and pentane, heavy gasoline including that, as a main component, such as benzene, and their intermediate medium gasoline (normal regular gasoline); therefore, if an ignition timing, etc., is not dully controlled corresponding to the characteristics of the gasoline, deterioration of driving performance or increase of harmful components included in the exhaust gas will occur.
In the USA and in European countries, in order to reduce the amount of petroleum oil consumption, fuel of gasoline mixed with alcohol becomes popular for automobiles, so that, by controlling the air-fuel ratio and the ignition timing corresponding to the alcohol content of the fuel, the air-fuel-ratio leaning can be prevented.
For a method of detecting characteristics of such fuel, a sensor being a type of measuring the refractive index of the fuel has been developed.
For example, a liquid-characteristics detection sensor has been disclosed in which, by inputting light from a light source into a short-period tilted grating, variation of a transmission-spectrum envelope shape fluctuating in accordance with a refractive index of material surrounding the grating under a cladding propagation mode, is analyzed using a signal analyzer (for example, Patent Document 1).
Further a liquid-characteristics detection sensor has been disclosed in which, by inputting light from a light source into a single short-period grating, variation of a transmission-spectrum shape fluctuating in accordance with a reflective index of material surrounding the grating under a cladding propagation mode is measured as a change in total receiving-light-amount, to thereby detect the reflective index (for example, Patent Document 2).    [Patent Document 1] International Publication WO 02/44697 (page 12 to 26, FIG. 3)    [Patent Document 2] International Publication WO 2006/126468 (page 6 to 9, FIG. 1)